1. Field of the Invention
The invention generally relates to gravel packing a well.
2. Description of the Prior Art
When well fluid is produced from a subterranean formation, the fluid typically contains particulates, or “sand.” The production of sand from the well must be controlled in order to extend the life of the well, and one technique to accomplish this involves routing the well fluid through a downhole filter formed from gravel that surrounds a sandscreen. More specifically, the sandscreen typically is a cylindrical mesh that is inserted into and is generally concentric with the borehole of the well where well fluid is produced. Gravel is packed between the annular area between the formation and the sandscreen, called the “annulus.” The well fluid being produced passes through the gravel, enters the sandscreen and is communicated uphole via tubing called a “wash pipe” that is inside of and concentric with the sandscreen.
The gravel that surrounds the sandscreen typically is introduced into the well via a gravel packing operation. In a conventional gravel packing operation, the gravel is communicated downhole via a slurry, which is a mixture of a carrier fluid and gravel. A gravel packing system in the well directs the slurry around the sandscreen so that when the fluid in the slurry disperses, gravel remains around the sandscreen.
A potential shortcoming of a conventional gravel packing operation is the possibly that carrier fluid may prematurely leave the slurry, either through the sandscreen or into the formation or both. When this occurs, a gravel plug commonly called a “bridge” forms in the slurry flow path, and this bridge forms a barrier that prevents slurry that is upstream of the bridge from being communicated downhole past the bridge. Thus, the bridge disrupts and possibly prevents the application of gravel around some parts of the sandscreen.
One type of gravel packing operation involves the use of a slurry that contains a high viscosity carrier fluid. Due to the high viscosity of this carrier fluid, the slurry may be communicated downhole at a relatively low velocity without significant fluid loss. However, the high viscosity fluid typically is expensive and may present environmental challenges relating to its use. Another type of gravel packing operation involves the use of a low viscosity fluid, such as a fluid primarily formed from water, in the slurry. The low viscosity fluid typically is less expensive than the high viscosity fluid. This results in a better quality gravel pack (leaves less voids in the gravel pack than high viscosity fluid) and may be less harmful to the environment. However, a potential challenge in using the low viscosity fluid is that the velocity of the slurry must be higher than the velocity of the high viscosity fluid-based slurry in order to prevent fluid from prematurely leaving the slurry.
A two-phase gravel packing operation has been used to distribute gravel around a sandscreen. The first phase involves gravel packing the well from the bottom up by introducing a gravel slurry flow into the annulus, as described above. If one or more bridges form during the first phase of the gravel packing operation, the gravel packing operation enters a second phase to circumvent these bridges in which the slurry flow is routed through alternative slurry flow paths commonly called “shunt tubes.” Such shunt tubes are, for example, disclosed in U.S. Pat. No. 7,147,054.
Even when using shunt tubes, the process of gravel packing is complicated by many factors including the friction pressure in long wash pipe sections (both wash-pipe/base-pipe annulus and wash-pipe itself) and the presence of potentially damaging formations such as shale formations. The friction pressure formation in the wash pipe sections can cause bottom hole pressure to exceed the fracturing pressure of the formation. Such a condition has negative side effects including: (1) potential loss of hydrostatic pressure creating a situation in which well control can be lost; (2) loss of expensive fluids to the formation either during or after the gravel packing process; and (3) loss of potentially damaging fluids to the formation either during or after the gravel pack operation. The presence of formations like reactive shale in the open hole can cause the fluid and gravel mixture to become contaminated with the shale and lead to damaged screens (plugged) and/or a damaged gravel pack.